Increase of antitumor activity of cisplatin using agonist of gonadotropin-realising hormone and inhibitor of aromatase on the model of ascites ovarian tumor

AIM

To study antitumor activity of triptorelin - agonist of gonadotropin-releasing hormone and exemestane - inhibitor of aromatase in monotherapy and in combination with cisplatin on the model of receptor-positive for estrogens and progesterone malignant ascites transplantable ovarian tumor (TOT), to assess therapeutic pathomorphosis and level of VEGF expression in tumor cells using diffe-rent combinations of cytostatics and hormonal drugs.

MATERIALS AND METHODS

72 female Wistar rats, which underwent intraperitoneal transplantation of ascitic TOT, by 5·10(6) cells per animal, have been involved in the study. Rats were divided into 8 groups, 9 rats in each group. Histological study with assessment of therapeutic pathomorphosis in TOT and immunohistochemical study has been carried out. Survival of animals in the studied groups has been evaluated.

RESULTS

Among animals treated in regimen of monotherapy, the most pronounced antiangiogenic activity in TOT has been observed on application of hormonal drugs (triptorelin - 39.4 ± 1.9 and exemestane - 33.9 ± 1.4%; р = 0.003), the highest grade of treatment pathomorphosis in TOT has been observed at treatment with cisplatin (11.7%; р = 0.001). Combination of triptorelin and exemestane has amplified antiangiogenic activity in TOT (12.2 ± 0.9%; р = 0.001), but has not significantly changed rates of pathomorphosis (22.1 ± 0.4%; р=0.005) and survival of animals (32.2%; р = 0.007) as compared with the same rates in rats treated with hormonal drugs in monotherapy. Significant correlation between VEGF expression and pathomorphosis has been established (relative part of viable tumor tissue (RPVTT)) in TOT (r = 0.712; р = 0.001), as well as between RPVTT and life-span of animals (r = -0.320; р = 0.007). However, lack of correlation between VEGF expression in cells of TOT and survival of rats has been determined (r = -0.194; р = 0.11). Combination of cytostatic agent with triptorelin or exemestane has demonstrated significantly high rates of therapeutic pathomorphosis (10.1 ± 0.1% and 16.2 ± 0.3%, respectively) and antiangiogenic activity in TOT (21.4 ± 1.4% and 15.0 ± 1.3%, respectively) as well as the highest survival of animals (100.0%, increase of life-span (ILS) = 231.9% and 85.7%, ILS = 205.8%, respectively) as compared with the same one in rats treated in regimen of monotherapy with cisplatin, triptorelin, exemestane or by combination of hormonal drugs. Among animals treated by combination of cytostatic drug with triptorelin, two were cured, and among rats, which received cisplatin and exemestane, one animal was cured.

CONCLUSIONS

Triptorelin and exemestane increase antitumor activity of cisplatin in respect to the malignant ascitic TOT and significantly increase survival of animals, especially when triptorelin and cisplatin are used in combination.

Langerhans cells facilitate epithelial DNA damage and squamous cell carcinoma

Polyaromatic hydrocarbons (PAHs) are prevalent, potent carcinogens, and 7,12-dimethylbenz[a]anthracene (DMBA) is a model PAH widely used to study tumorigenesis. Mice lacking Langerhans cells (LCs), a signatory epidermal dendritic cell (DC), are protected from cutaneous chemical carcinogenesis, independent of T cell immunity. Investigation of the underlying mechanism revealed that LC-deficient skin was relatively resistant to DMBA-induced DNA damage. LCs efficiently metabolized DMBA to DMBA-trans-3,4-diol, an intermediate proximal to oncogenic Hras mutation, and DMBA-treated LC-deficient skin contained significantly fewer Hras mutations. Moreover, DMBA-trans-3,4-diol application bypassed tumor resistance in LC-deficient mice. Additionally, the genotoxic impact of DMBA on human keratinocytes was significantly increased by prior incubation with human-derived LC. Thus, tissue-associated DC can enhance chemical carcinogenesis via PAH metabolism, highlighting the complex relation between immune cells and carcinogenesis.

Eugenol inhibit 7,12-dimethylbenz[a]anthracene-induced genotoxicity in MCF-7 cells: Bifunctional effects on CYP1 and NAD(P)H:quinone oxidoreductase

Typically, chemopreventive agents either inhibit the cytochrome P450s (CYPs) that are essential for the metabolism of carcinogens or induce phase II detoxifying enzymes. This study examined the chemopreventive effect of eugenol on 7,12-dimethylbenz[a]anthracene (DMBA)-induced DNA damage in MCF-7 cells. Eugenol inhibited the formation of the DMBA-DNA adduct in a dose dependent manner. CYP1A1 and CYP1B1 activity, which catalyze the biotransformation of DMBA, were strongly inhibited by eugenol. Eugenol also suppressed the CYP1A induction by DMBA through decreased aryl hydrocarbon receptor activation and subsequent DNA binding. Furthermore, eugenol increased the expression and activity of NAD(P)H:quinone oxidoreductase (QR), a major detoxifying enzyme for DMBA, through NF-E2 related factor2 binding to antioxidant response element in QR gene. Therefore, eugenol has a potent protective effect against DMBA-induced genotoxicity, presumably through the suppression of the DMBA activation and the induction of its detoxification. These results suggest that eugenol has potential as a chemopreventive.

Anti-tumor promoting potential of luteolin against 7,12-dimethylbenz(a)anthracene-induced mammary tumors in rats

In this study, we evaluated the anti-tumor potential of luteolin (30mg/kg, p.o.), combined with cyclophosphamide (10mg/kg, i.p.) (LU+CYC) orally administered for 20 days; and CYC individually for 10 days against 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinogenesis in Wistar rats. Combination treatment (LU+CYC) inhibited the incidence rate of tumors and decreased tumor volume significantly without changing the total body weight of the animals. Long-term treatment did not show any apparent toxicity in rats. The CYC-treated group showed potential reduction of tumor volume (74%), severe toxicity, and loss of body weight. In order to elucidate the anticancer mechanism of luteolin, antioxidant activities such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) generation in the liver, kidney and breast, as well as protein profiles, were also examined. Biochemical analysis of the combination-treated group showed significant (P<0.01; P<0.05) inhibition of lipid peroxide (LPx) formation (oxygen-free radicals), the level and the activity of SOD, CAT and GPx were found to be very high than the LU and CYC individually treated rats at a 30mg/kg dose. 2D gel electrophoresis analysis revealed that (56kDa) high molecular weight protein was detected in tumors of rats receiving combination treatment than the cancer controls. The biological significance of that protein involved for the dysfunction of cancer cells and induces apoptosis. Histopathological changes also confirmed the formation of tumor tubules and neovascularization after the treatment. Overall, these results suggest that the combination treatment provided antioxidant defense with strong chemopreventive activity against the genesis of DMBA-induced mammary tumors.

The aspirin metabolite, salicylate, inhibits 7,12-dimethylbenz[a]anthracene-DNA adduct formation in breast cancer cells

There is evidence that aspirin and other non-steroidal anti-inflammatory drugs may be protective agents against cancer in the gastrointestinal tract. These effects are particularly well documented for the colon and rectum. Some epidemiological and experimental studies have suggested that aspirin could also be a chemopreventive agent against breast cancer. We investigated the effects of the aspirin metabolite, salicylate (SA), on 7,12-dimethylbenz[a]anthracene (DMBA)-DNA adduct formation as well as on the expression of the enzymes involved in the carcinogen bioactivation pathway, in particular cytochrome P450 1A (CYP1A) and cyclooxygenases (COX-1 and COX-2). The effects of the test drug were examined in both the human mammary carcinoma cell line, MCF-7, and mammary cells derived from DMBA-induced rat mammary tumours (RMTCs). In this study, we also reported the effects of SA on cell growth and viability in breast cancer cells (BCCs). The results demonstrated that DMBA-DNA adduct formation in both cancer cell lines was inhibited by SA at concentrations of > or = 2.5 mM. CYP1A was undetectable in RMTCs while CYP1A induction by beta-naphthoflavone in MCF-7 cells was significantly inhibited by SA in a concentration-dependent manner. Aspirin did not affect COX-1 expression in either of the BCCs. COX-2 was not detected in MCF-7 cells, but its expression in RMTCs was inhibited by SA treatment, which also significantly reduced BCC growth, but failed to cause cell death by necrosis or apoptosis. These data suggest that inhibition of DMBA-DNA adduct formation may contribute to aspirin breast cancer chemopreventive action and indicate that this drug can act in the first stage of carcinogenesis.

Suppression of 7,12-dimethylbenz[a]anthracene-induced mammary carcinogenesis by pre-initiation treatment of rats with beta-naphthoflavone coincides with decreased levels of the carcinogen-derived DNA adducts in the mammary gland

BACKGROUND

Mechanisms underlying prevention by beta-naphthoflavone (beta-NF) of mammary carcinogenesis initiated with 7,12-dimethylbenz[a]anthracene (DMBA) in the rat were elucidated.

METHODS AND RESULTS

Treatment of female Sprague-Dawley rats with beta-NF at 40 mg/kg b.wt. for 4 days by oral gavage in corn oil before a single oral dose of DMBA (112 mg/kg b.wt.) suppressed mammary gland carcinogenesis as shown by an increase in the median latent period from 10 to 24 weeks and a 60% decrease in the multiplicity of mammary adenocarcinomas. In contrast, a 20-day treatment with beta-NF starting 3 weeks after DMBA had no significant effects on mammary tumorigenesis. The activities of phase I and phase II enzymes were examined in the liver and mammary gland 24 h after treatment of rats with beta-NF, DMBA, or beta-NF followed by DMBA as in the first bioassay. Treatment with either beta-NF or DMBA increased the hepatic activities of cytochrome P450 (CYP)1A1, 1A2, and 2B1/2, and glutathione S-transferase, and the mammary activity of CYP1A1. The activity of mammary CYP2B1/2 induced by DMBA was decreased by beta-NF. In the liver, the increase of UDP-glucuronosyl transferase (GT) activity in rats treated with beta-NF and DMBA was 2.3-fold greater than in rats treated with DMBA alone. Thus, treatment with beta-NF likely increased the rate of glucuronidation of DMBA dihydrodiols leading to carcinogen detoxification. The levels of the DMBA adducts determined by 32P-postlabeling of the mammary gland DNA were decreased in the beta-NF-pretreated rats.

CONCLUSION

The beta-NF-induced increase in the hepatic UDP-GT activity and decrease in the mammary DNA-DMBA adducts occurred under the same treatment regimen that led to suppression of DMBA-induced mammary carcinogenesis.

Purple grape juice inhibits 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumorigenesis and in vivo DMBA-DNA adduct formation

There has been considerable interest in identifying specific foods and phytochemicals that may have breast cancer preventive properties. Concord grapes are rich in polyphenolic chemicals and anthocyanin pigments that may have biological properties which could suppress cancer such as having antioxidant, antiproliferative, and proapoptotic actions. To determine the potential breast cancer protective action of purple grape juice, we examined the effect of grape juice consumption on the initiation stage of 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumorigenesis and on the in vivo formation of rat mammary DNA adducts in female Sprague-Dawley rats. Consumption of grape juice significantly inhibited mammary tumor mass at termination and the growth of tumors for the first 5 weeks of detectable tumor development. Consumption of grape juice phenolics by rats also significantly inhibited in vivo mammary DMBA-DNA adduct formation by 34 and 56% for animals fed phenolics at 346 and 692 mg/dL, respectively, compared to controls. Mammary 8-oxo-deoxyguanosine (8-oxo-dG) levels decreased by 25 and 37%, respectively, but the differences were not statistically significant. Liver DMBA-DNA adducts decreased by 10-30%, while 8-oxo-dG adducts remained unchanged, following grape juice intake. Liver glutathione S-transferase activity was significantly increased following grape juice consumption, but only at the highest level of intake. In addition, liver activities of catalase increased and xanthine oxidase decreased significantly, but only at the highest grape juice dose. Thus, these studies indicate that specific constituents or combinations of phytochemicals in purple grape juice can block the initiation stage of DMBA-induced rat mammary tumorigenesis. This tumor inhibitory effect was associated with a suppression of mammary DMBA-DNA adduct formation, which in part may be explained by increased liver activity of the phase II metabolizing enzyme, glutathione S-transferase. Mammary and liver 8-oxo-dG levels were not significantly altered by grape juice consumption. Thus, grape juice constituents appear to have benefit in decreasing susceptibility of the rat mammary gland to the tumor-initiating action of DMBA.

Naturally occurring coumarins inhibit 7,12-dimethylbenz[a]anthracene DNA adduct formation in mouse mammary gland

Naturally occurring coumarins (NOCs) are anti-carcinogenic in the mouse skin model. To characterize the chemopreventive potential of NOCs against breast cancer, we first examined their effects on 7,12-dimethylbenz[a]anthracene (DMBA)-DNA adduct formation in mouse mammary gland. We hypothesized that those NOCs that both inhibited cytochrome P450 1A1/1B1 and induced hepatic glutathione S-transferases (GSTs) would be the most effective in blocking DMBA-DNA adduct formation in mouse mammary gland. To address this hypothesis, simple coumarins (e.g. coumarin and limettin, which induced mouse hepatic GSTs but had little effect on P4501A1/1B1) and linear furanocoumarins (e.g. imperatorin and isopimpinellin, which induced hepatic GSTs and were potent inhibitors of P4501A1/1B1) were compared. Mice were pretreated with NOCs (150 mg/kg body wt, by gavage) prior to either a single dose of DMBA (50 microg) or multiple doses of DMBA (20 microg daily for 3 and 6 weeks). Mammary DMBA-DNA adduct formation was quantitated by the nuclease P1-enhanced 32P-postlabeling assay. With the single dose of DMBA, coumarin, limettin, imperatorin and isopimpinellin inhibited DMBA-DNA adduct formation by 50, 41, 79 and 88%, respectively. Coumarin, limettin and imperatorin blocked DMBA-DNA adduct formation by 36, 60, and 66% at 3 weeks, and by 0, 49 and 55% at 6 weeks of DMBA dosing, respectively. In a 6 week dose-response study of select NOCs and 7,8-benzoflavone (a potent P4501 inhibitor that had little effect on GSTs), DMBA-DNA adduct formation was inhibited by 0, 43 and 24% in the limettin groups; by 26, 26 and 69% in the isopimpinellin groups; and by 80, 96 and 97% in the 7,8- benzoflavone groups at 35, 70 and 150 mg/kg, respectively. Taken together, these results suggest that linear furanocoumarins had a greater inhibitory effect on DMBA-DNA adduct formation in mouse mammary glands compared with simple coumarins, and that the predominant effect may be P4501 inhibition.

Combined efficacy of tamoxifen and coenzyme Q10 on the status of lipid peroxidation and antioxidants in DMBA induced breast cancer

An increasing amount of experimental and epidemiological evidence implicates the involvement of oxygen derived radicals in the pathogenesis of cancer development. It is well known that chemical carcinogenesis is multistage process. Free radicals arefound to be involved in both initiation and promotion of multistage carcinogenesis. Tamoxifen (TAM) is a potent antioxidant and a non-steroidal antiestrogen drug most used in the chemotherapy and chemoprevention of breast cancer. Besides its anticarcinogenic potential, it also produces some adverse toxic side effects, while taken for a long time. In order to minimise the side effects and to improve the antioxidant efficacy of tamoxifen, coenzyme Q10 (CoQ10) was added. Hence the present study was designed to investigate the combined efficacy of TAM along with CoQ10 in 7, 12 dimethyl benz(a)anthracene (DMBA) induced peroxidative damage in rat mammary carcinoma. The experimental setup comprised of one control and five experimental groups and it was carried out in adult female Sprague-Dawley rats. Mammary carcinoma was induced by oral administration of DMBA (25 mg kg(-1) body wt) and the treatment was started by the oral administration of TAM (10 mg kg(-1) body wt day(-1)) and CoQ10 (40 mg kg(-1) body wt day(-1)) dissolved in olive oil and continued for 28 days. Rats induced with DMBA showed a decline in the thiol capacity of the cell accompanied by high malondialdehyde content levels along with lowered activities of antioxidant status (superoxide dismutase, catalase, glutathione peroxidase and reduced glutathione). In contrast, glutathione metabolising enzymes (glutathione reductase, glucose-6-phosphate dehydrogenase and glutathione-S-transferase) were increased significantly in chemically induced carcinoma bearing rats. Administration of TAM along with CoQ10 restored the activities to a significant level thereby preventing cancer cell proliferation. This study highlights the increased antioxidant enzyme activities in relation to the susceptibility of cells to carcinogenic agents and the response of tumour cells to the chemotherapeutic agents.

Rapid induction of mammary cancer by repeated subcutaneous injection of the trans-3,4-dihydrodiol of 7,12-dimethylbenz[a]anthracene in the female Sprague–Dawley rat

The oral administration of a single 20mg dose of 7,12-dimethylbenz[a]anthracene regularly and rapidly induces mammary cancer in 50 day-old Sprague-Dawley female rats [Experimental Leukemia and Mammary Cancer, 1979, p. 74]. Several mechanisms by which 7,12-dimethylbenz[a]anthracene induces mammary cancer have been proposed and various derivatives have been implicated as possible proximate or ultimate electrophilic and carcinogenic forms of this hydrocarbon. Here we show that 7,12-dimethylbenz[a]anthracene-trans-3,4-dihydrodiol rapidly induces mammary cancer by repeated subcutaneous injection in a high proportion of female Sprague-Dawley rats without malignancies at the site of injection, whereas its more lipid soluble diacetate derivative induced injection site sarcomas in addition to distal mammary cancers. By contrast, repeated subcutaneous injection of 7,12-dimethylbenz[a]anthracene and its 7-meso-aldehyde derivative induced subcutaneous sarcoma in most, if not all, rats and a few mammary cancers.

Augmented efficacy of tamoxifen in rat breast tumorigenesis when gavaged along with riboflavin, niacin, and CoQ10: Effects on lipid peroxidation and antioxidants in mitochondria

Reactive oxygen species (ROS) play a major role in causing mitochondrial changes linked to cancer and metastasis. Uptake of antioxidants by tissue to reduce the ROS production could be instrumental in controlling cancer. Tamoxifen (TAM), a nonsteroidal anti-estrogen drug most used in the chemotherapy and chemoprevention of breast cancer. Riboflavin, niacin and coenzyme Q10 (CoQ10) are proved to be potent antioxidants and protective agents against many diseases including cancer. The objective of this research is to determine the therapeutic efficacy of combinatorial therapy on mammary carcinoma bearing rats in terms of the mitochondrial lipid peroxidation and antioxidant status especially MnSOD. Female albino rats of Sprague-Dawley strain were selected for the investigation. Mammary carcinoma was induced with 7,12-dimethyl benz(a)anthracene (DMBA: 25 mg), and the treatment was started by the oral administration of TAM (10 mg/kg body weight/day) along with riboflavin (45 mg/kg body weight/day), niacin (100 mg/kg body weight/day) and CoQ10 (40 mg/kg body weight/day) for 28 days. The levels of lipid peroxides, activities of enzymic and non-enzymic antioxidants were measured in the mitochondria isolated from the mammary gland and liver of control and experimental rats. Rats treated with DMBA showed an increase in mitochondrial lipid peroxidation (mammary gland 52.3%; liver 25.1%) accompanied by high malondialdehyde levels along with lowered activities of mitochondrial enzymic antioxidants [superoxide dismutase (mammary gland 19.9%; liver 24.8%), catalase (mammary gland 50%; liver 19.7%), glutathione peroxidase (mammary gland 47.8%; liver 31.1%)] and non-enzymic antioxidants [reduced glutathione (mammary gland 14.3%; liver 13.3%), Vitamin C (mammary gland 6.49%; liver 21.4%) and E (mammary gland 20.3%; liver 22.2%)]. Administration of combinatorial therapy restored lipid peroxide level and the activities of enzymic and non-enzymic antioxidants to near normalcy. In addition, antitumour activity was also found to be enhanced which is evident from the increased expression of tumour suppressor gene MnSOD thereby preventing cancer cell proliferation. These results suggested that TAM treatment is the most effective during co-administration of riboflavin, niacin and CoQ10 in terms of mitochondrial antioxidant and antitumour activity.

Alteration in phase I and II enzyme activities and polycyclic aromatic hydrocarbons-DNA adduct formation by plant phenolics in mouse epidermis

Several naturally occurring plant phenols were shown to inhibit the mutagenicity and/or tumorigenicity of chemical carcinogens, including polycyclic aromatic hydrocarbons (PAHs). In this study, the effect of the topical application of three structurally diverse phenolic acids and trihydroxystilbene, resveratrol, on epidermal aryl hydrocarbon hydroxylase (AHH), phase II enzymes, as well as the binding of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA) to epidermal DNA were compared. The single, topical application of 8 and 16 mumol of protocatechuic or chlorogenic acid increased the activity of AHH by 10-30%, whereas resveratrol in a dose of 16 mumol almost completely (99%) inhibited the enzyme activity. Phenolic acids also increased the activities of phase II enzymes. Resveratrol did not affect the glutathione S-transferase activity but induced UDP glucuronosyltransferase (by approximately 100-150%) and to a lesser extent NAD(P)H:quinone oxidoreductase. In a dose of 16 micromol all phenolic acids afforded 40-50% inhibition of covalent benzo[a]pyrene-diol-epoxide (B[a]PDE) binding to DNA. Resveratrol had no effect on B[a]PDE adduct formation but reduced the levels of all the major DMBA adducts. Phenolic acids, particularly tannic acid, mostly affected the formation of syn- and anti-DMBADE dAdo adducts. These results indicate that both the modulation of carcinogen activating enzymes and the prevention of their ultimate metabolites binding to DNA by naturally occurring phenolics are involved in the antitumorigenic activity of these compounds. For phenolic acids, however, their interactions with reactive PAH metabolites and/or blocking of a specific binding site in a genome seem more important. Derivatives of stilbene, such as resveratrol, affect DNA adduct formation and thus the initiation of tumorigenesis through the interaction with the Ah receptor rather than the scavenging active metabolites.

Mass spectrometric analysis of 7-sulfoxymethyl-12-methylbenz[a]anthracene and related electrophilic polycyclic aromatic hydrocarbon metabolites

The Meso-region theory of polycyclic aromatic hydrocarbon (PAH) carcinogenesis predicts that the development of pronounced carcinogenicity depends on the introduction of a good leaving group on alkyl side-chains attached to the exceptionally reactive meso-anthracenic or L-region positions of PAHs. Thus, the first step in carcinogenesis by methylated PAHs such as 7,12-dimethylbenz[a]anthracene (DMBA) would be the hydroxylation of the L-region methyl groups, particularly the 7-methyl group. The second would be the formation of a metabolite, e.g. a sulfate ester, which is expected to be a good leaving group capable of generating a highly reactive benzylic carbocation. 7-Hydroxymethyl-12-methylbenz[a]anthracene (7-HMBA) is a metabolite of DMBA, and sulfation of 7-HMBA to a 7-sulfoxymethyl metabolite (7-SMBA) is a known Phase II metabolic process designed to facilitate excretion, but actually enabling more destructive side-reactions. These side-reactions occur with generation of an electrophilic 7-methylene carbonium ion, and/or by in vivo halide exchange to provide neutral side-products more capable of entering cells, especially those of DMBA target tissues. Electrospray ionization mass spectrometry (MS) enabled us to visualize 7-SMBA as an intact m/z 351 conjugate anion by negative mode, and as a released m/z 255 carbonium ion by positive mode. Upon prolonged refrigeration, 7-SMBA accumulated an m/z 383 photooxide, which appeared capable of re-evolving the starting material as visualized by tandem quadrupole MS, or MS/MS. The 7-SMBA carbonium ion provided interpretable fragments when studied by fragment ion MS/MS, including those representing the loss of up to several protons. Subtle differences in this property were encountered upon perturbing 7-SMBA, either by warming it at 37 degrees C for 2 h or by substituting the initial sulfoxy group with an iodo group. Side-reactions accounting for such proton losses are proposed, and are of interest whether they occur in the mass spectrometer, in solution or both; these proposals include acidity at the 12-methyl position and cyclization between the 12-methyl group and the adjacent C-1 position. It is also suggested that such side-reactions may comprise one route to relieving steric strain arising between the 12-methyl group and the angular benzo ring of 7-SMBA.

Mechanisms of mammary cancer chemoprevention by organoselenium compounds

Searching for optimal diets and for naturally occurring agents in routinely consumed foods that may inhibit cancer development, although challenging, constitutes a valuable and plausible approach to finding ways to control and prevent cancer. To date, the use of the micronutrient selenium in human clinical trials is limited but the outcome of these investigations indicates that selenium is one of the most promising agents. Data presented in this mini-review indicate that the dose and the form (structure) in which selenium is used are the most critical determinants of success in future clinical trials. The focus of this mini-review is on the mechanisms of mammary cancer chemoprevention by organoselenium compounds. Among the naturally occurring organoselenium compounds, Se-Methylselenocysteine is more efficacious than the most extensively studied forms, such as selenomethionine. However, we showed that synthetic organoselenium compounds can be tailored to achieve greater chemopreventive efficacy with minimal side effects by structural modifications; it is evident that synthetic agents are superior to the inorganic selenite, naturally occurring selenium compounds and their sulfur-containing analogs. We have demonstrated that 1,4-phenylenebis (methylene) selenocyanate (p-XSC) and its putative metabolite glutathione conjugate (p-XSeSG) are highly promising agents in the chemoprevention of mammary carcinogenesis in the 7,12-dimethylbenz[a]anthracene (DMBA)-rat mammary tumor model system. Both compounds inhibit the initiation phase of carcinogenesis by inhibiting DMBA-DNA adduct formation in the target organ in vivo. cDNA microarray analysis indicates that both selenium compounds alter genes in a manner that leads to inhibition of cell proliferation and induction of apoptosis; modulation of apoptosis and cell proliferation can account for chemoprevention during the post-initiation phase of mammary carcinogenesis. Using a rat mammary cancer cell line, we compared p-XSC and p-XSeSG as inhibitors of cell proliferation; depending on the selenium dose and time point selected, p-XSC was comparable to or better than p-XSeSG. Collectively, the results described here, suggest that the molecular targets modulated by organoselenium compounds are highly useful indicators of success in clinical cancer chemoprevention trials.

Comparative action of 1,4-phenylenebis(methylene)selenocyanate and its metabolites against 7,12-dimethylbenz[a]anthracene-DNA adduct formation in the rat and cell proliferation in rat mammary tumor cells

1,4-phenylenebis(methylene)selenocyanate (p-XSC) inhibits 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary carcinogenesis and DMBA-DNA binding in the rat mammary gland. Tetraselenocyclophane (TSC) was identified in rat feces as a metabolite of p-XSC. This led us to postulate the metabolic pathway: p-XSC-->glutathione conjugate (p-XSeSG)-->aromatic selenol (p-XSeH)-->TSC. Whether p-XSC or one of its metabolites is responsible for cancer prevention is the focus of this study. We utilized the DMBA-DNA binding assay with p-XSC as a positive control to evaluate the chemopreventive potential of p-XSC metabolites at dietary selenium levels of 10 ppm. Rats were fed AIN-76A diet supplemented with various selenium compounds for 1 week prior to the oral administration of a single dose of [3H]DMBA (5 mg per rat, specific activity 51.3 mCi/mmol). The rats were sacrificed 24 h later and DNA was isolated from the mammary fat pads. Relative levels of total binding were: [pmol/mg DNA, mean +/- S.D., n=6]; DMBA [7.2 +/- 1.6]; DMBA+p-XSC [3.5 +/- 2.7]; DMBA+p-XSeSG [2.2 +/- 1.1]; DMBA+TSC [5.6 +/- 2.9]. All selenium compounds, except TSC, significantly inhibited DMBA-DNA adduct formation; however, the difference between p-XSC and p-XSeSG was not statistically significant. The inhibition of total binding was attributed to a reduction in the formation of the three major adducts derived from bay-region diol epoxides of DMBA. On the basis of their chromatographic characteristics, these were identified as anti-diol-epoxide:deoxyguanosine, syn-diol-epoxide:deoxyadenosine, and anti-diol-epoxide:deoxyadenosine. Our results suggest that p-XSeSG, but not TSC, is the likely inhibitor of mammary cancer. Selenium levels measured by atomic absorption spectroscopy in the target organ (mammary fat pads) and in plasma following the dietary administration of selenium compounds were in the order of p-XSeSG congruent with p-XSC>TSC. These results appear to be consistent with their order of inhibitory effects on total DMBA-DNA binding. Further in vitro studies of the effect of selenium compounds on cell proliferation suggest that, depending on the dose and time point selected, p-XSC is comparable to or better than p-XSeSG; but both are more effective than TSC. Collectively, our in vivo and in vitro results indicate that p-XSC and its conjugate are better candidates than TSC for future studies on mammary cancer chemoprevention.

The effect of plant phenolics on the formation of 7,12-dimethylbenz[a]anthracene-DNA adducts and TPA-stimulated polymorphonuclear neutrophils chemiluminescence in vitro

Phenolics, common plant constituents, form up an important part of human diet and are considered potential chemopreventive agents. In the present study, structurally diverse phenolics, such as tannic acid, protocatechuic acid, chlorogenic acid and resveratrol, were investigated for their inhibitory effects on covalent binding of 7,12-dimethylbenz[a]anthracene (DMBA) to DNA in vitro and the suppression of oxidative burst in 12-O-tetradecanoylphorbol-13-acetate (TPA)-stimulated human polymorphonuclear neutrophils (PMNs). 32P-postlabeling analysis of DNA incubated with DMBA in the presence of 3-methylcholanthrene (3-MC)-induced microsomes produced three major adducts derived from anti-, syn- and anti-dihydrodiol epoxides through reactions with dGuo and dAdo, respectively. Phenolic compounds at the concentration of 150 microM reduced the levels of all DMBA-DNA adducts by 55-98%. The most dramatic effect was observed in case of tannic acid, which completely inhibited the formation of DMBA-dAdo adducts. Chlorogenic acid was the least effective inhibitor of DMBA-DNA adducts formation particularly syn-DMBADE-dAdo (20%). Human neutrophils showed a significant dose-related decrease of TPA-induced chemiluminescence after pretreatment with phenolic compounds. The most effective inhibitors were tannic acid and resveratrol with IC(50)=5.19 and 5.76 microM, respectively. These results suggest that the suppression of reactive oxygen species (ROS) and carcinogen-DNA adducts formation may be important for anticarcinogenic activity of the examined phenolics.

Detection of benzylic adducts in DNA and nucleotides from 7-sulfooxymethyl-12-methylbenz[a]anthracene and related compounds by 32P-postlabeling using new TLC systems

7,12-Dimethylbenz[a]anthracene (DMBA) is a highly potent experimental carcinogen, that must be transformed to its ultimate carcinogenic form in vivo. The meso-region theory of aromatic hydrocarbon carcinogenesis predicts that 7-hydroxymethyl sulfate (7-HMBA) ester plays a major role in the metabolic activation, benzylic DNA adduct formation and complete carcinogenicity of HMBA and DMBA. This study was undertaken to detect highly lipophilic benzylic DNA adducts resulting from the reaction between 7-hydroxymethy sulfate ester of HMBA (7-SMBA) and DNA as well as determine their DNA base selectivity. Synthetic 7-SMBA was incubated with DNA (800 microg/ml) and individual deoxynucleoside 3'-monophosphates (600 microg/ml) and benzylic adducts were analyzed by 32P-postlabeling/TLC following their enrichment with butanol extraction. Dilute ammonium hydroxide-based solvents were developed to detect the highly lipophilic aralkyl adducts. The reaction with DNA, dGp and dAp gave rise to multiple adducts; dCp and dTp showed no significant adducts. Chromatographic comparison revealed that the major DNA adduct was derived from dG. The methodology developed was also found applicable for highly lipophilic adducts resulting from sulfate esters of structurally-related metabolites of DMBA.

Naturally occurring coumarins inhibit human cytochromes P450 and block benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene DNA adduct formation in MCF-7 cells

Naturally occurring coumarins (NOCs) inhibit polycyclic aromatic hydrocarbon-induced skin tumor initiation in mice by blocking cytochrome P450 (P450)-mediated bioactivation of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA). Bergamottin selectively inhibits tumor initiation by B[a]P, whereas imperatorin and isopimpinellin inhibit tumor initiation with both carcinogens. The goals of the current study were to examine the ability of NOCs to inhibit human P450s in vitro and to establish whether NOCs, which are anticarcinogenic in mice, can block carcinogen bioactivation in cultured human cells. For the initial experiments, incubations containing 5 microM P450, P450 substrate, an NADPH generating system, and NOCs were used to determine the concentrations of each inhibitor that blocked 50% of P450 activity (IC(50)). These results confirmed that NOCs are capable of inhibiting multiple human P450s and that they exhibit selectivity for certain isoforms of human P450s. In subsequent experiments, we examined the effects of bergamottin, imperatorin, and isopimpinellin on DMBA and B[a]P DNA adduct formation in the human breast MCF-7 adenocarcinoma cell line. Coincubation of cells with the three different NOCs significantly inhibited DMBA DNA adduct formation by 29-82% at doses ranging from 2 to 10 microM and significantly inhibited B[a]P DNA adduct formation by 37-80% at doses ranging from 20 to 80 microM. HPLC analysis of the DNA hydrolysates demonstrated that inhibition of DNA adducts corresponded to inhibition of the major B[a]P and DMBA diol-epoxide-derived adducts. Although bergamottin was not effective at blocking DMBA bioactivation in the mouse skin model, it was similar in effectiveness to imperatorin and isopimpinellin in MCF-7 cells. These results demonstrate that NOCs, which are present in citrus fruits and other components of the human diet, are capable of inhibiting carcinogen metabolizing enzymes and blocking bioactivation of both B[a]P and DMBA in MCF-7 cells.

Baicalein inhibits DMBA-DNA adduct formation by modulating CYP1A1 and CYP1B1 activities

Flavonoids are phenolic compounds isolated from plants, and several of them like genistein and quercetin, have been documented to be effective in preventing cancer. Baicalein, a flavonoid extracted from the root of Scutellaria species, is widely used as a health supplement and herbal medicine in Asian countries. In this study, the chemopreventive effect of baicalein on 7,12-dimethylbenz[a]anthracene (DMBA)-induced DNA damage was evaluated in an established cell culture model. In a preliminary screening, baicalein was identified to be a strong inhibitor to EROD activities induced by DMBA in MCF-7 cells. Subsequent enzyme kinetic analysis revealed that baicalein was a competitive inhibitor to EROD, and CYP1A1 and CYP1B1 gene expressions were also determined. Baicalein could reduce the CYP1A1/1B1 mRNA expressions induced by DMBA, and the mRNA abundance of CYP1A1 appeared to be more responsive than that of CYP1B1. A XRE-luciferase gene reporter assay indicated that AhR transactivation was suppressed. Since CYP1A1/1B1 were responsible for the biotransformation of polycyclic aromatic hydrocarbons, baicalein also demonstrated its ability to reduce DMBA-DNA adduct formation in MCF-7 cells. This study suggested that the natural occurring baicalein could be an agent preventing carcinogen-DNA adduct formation.

Resistance of MCF-7 cells to dimethylbenz(a)anthracene-induced apoptosis is due to reduced CYP1A1 expression

We have developed a series of aryl hydrocarbon (AH)-resistant cell lines derived from MCF-7 human breast epithelial cancer cells by continuous exposure to the AH benzo[a]pyrene. These cell lines display cross-resistance to the mammary carcinogen dimethylbenz[a]anthracene (DMBA). Apoptosis induced by exposure to DMBA is greatly decreased in the resistant cell lines compared to the wild-type, in proportion to the level of resistance. Apoptosis induced by DMBA could be blocked by inhibitors of DMBA metabolism such as alpha-naphthoflavone and diosmetin. We therefore examined the resistant cell lines for their ability to metabolize DMBA and for the formation of DMBA-DNA adducts, and found that both parameters were decreased compared to wild-type cells in proportion to the level of resistance. When exposed to DMBA or 2,3,7,8-tetrachlorodibenzo-p-dioxin, the resistant cell lines have a diminished capacity to carry out ethoxyresorufin-O-deethylation, indicating that the induction of cytochrome P450 1A1 (CYP1A1) enzyme is impaired. We therefore examined the expression of the CYP1A1 gene, and found reduced levels of both CYP1A1 mRNA and CYP1A1-promoter controlled transcription in resistant cells compared to the wild-type. The deleterious effects of AHs are believed to be mediated by the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor which regulates CYP1A1 expression. Resistant cell lines had a reduced expression of the AhR, as measured at the mRNA and protein levels. These data demonstrate that AH resistance in these cells is mediated by changes in the signal transduction pathway which regulates CYP1A1 expression.

Deficiency of either cyclooxygenase (COX)-1 or COX-2 alters epidermal differentiation and reduces mouse skin tumorigenesis

Nonsteroidal anti-inflammatory drugs are widely reported to inhibit carcinogenesis in humans and in rodents. These drugs are believed to act by inhibiting one or both of the known isoforms of cyclooxygenase (COX). However, COX-2, and not COX-1, is the isoform most frequently reported to have a key role in tumor development. Here we report that homozygous deficiency of either COX-1 or COX-2 reduces skin tumorigenesis by 75% in a multistage mouse skin model. Reduced tumorigenesis was observed even though the levels of stable 7,12-dimethylbenz(a)anthracene-DNA adducts were increased about 2-fold in the COX-deficient mice compared with wild-type mice. The premature onset of keratinocyte terminal differentiation appeared to be the cellular event leading to the reduced tumorigenesis because keratin 1 and keratin 10, two keratins that indicate the commitment of keratinocytes to differentiate, were expressed 8-13-fold and 10-20-fold more frequently in epidermal basal cells of the COX-1-deficient and COX-2-deficient mice, respectively, than in wild-type mice. Papillomas on the COX-deficient mice also displayed the premature onset of keratinocyte terminal differentiation. However, loricrin, a late marker of epidermal differentiation, was not significantly altered, suggesting that it was the early stages of keratinocyte differentiation that were primarily affected by COX deficiency. Because keratin 5, a keratin associated with basal cells, was detected differently in papillomas of COX-1-deficient as compared with COX-2-deficient mice, it appears that the isoforms do not have identical roles in papilloma development. Interestingly, apoptosis, a cellular process associated with nonsteroidal anti-inflammatory drug-induced inhibition of tumorigenesis, was not significantly altered in the epidermis or in papillomas of the COX-deficient mice. Thus, both COX-1 and COX-2 have roles in keratinocyte differentiation, and we propose that the absence of either isoform causes premature terminal differentiation of initiated keratinocytes and reduced tumor formation.

Relative importance of maternal and embryonic microsomal epoxide hydrolase in 7,12-dimethylbenz[a]anthracene-induced developmental toxicity

Microsomal epoxide hydrolase (mEH) catalyzes the hydrolysis of epoxide intermediates derived from drugs and environmental chemicals. The response of in vivo (embryo) and in vitro (embryo fibroblast) tests were analyzed using mEH-null and wild-type mice to determine the relative role of maternal and embryonic mEH in the developmental toxicity induced by 7,12-dimethylbenz[a]anthracene (DMBA). Embryos derived from DMBA-treated [50mg/kg, daily from gestational day (GD) 11 to GD 15] dams were analyzed. Although weight (P=0.0009) and crown-rump length (P=0.0003) of wild-type fetuses on GD 18 were significantly lower than those of mEH-null fetuses, respectively, no significant difference was found between mEH-null and heterozygous fetuses of mEH-null dams. Cell viability was decreased to 50% in wild-type mouse embryo fibroblasts (MEFs) treated with 3 microM DMBA, but no significant decrease was found in mEH-null MEFs. DMBA-3,4-diol produced a significant decrease in cell viability and suppressed the proliferation of wild-type MEFs at a 10-fold lower concentration than did DMBA. Although mEH protein was expressed in liver microsomes from wild-type embryos (GD 15), DMBA-3,4-diol was not detected among the DMBA metabolites. However, it was detected in the serum of wild-type pregnant mice treated with DMBA, but not in that of mEH-null mice. These results suggest that maternal mEH plays a major role in DMBA-induced developmental toxicity, and embryonic mEH is less involved in the toxicity.

Aromatic hydrocarbon receptor-driven Bax gene expression is required for premature ovarian failure caused by biohazardous environmental chemicals

Polycyclic aromatic hydrocarbons (PAHs) are toxic chemicals released into the environment by fossil fuel combustion. Moreover, a primary route of human exposure to PAHs is tobacco smoke. Oocyte destruction and ovarian failure occur in PAH-treated mice, and cigarette smoking causes early menopause in women. In many cells, PAHs activate the aromatic hydrocarbon receptor (Ahr), a member of the Per-Arnt-Sim family of transcription factors. The Ahr is also activated by dioxin, one of the most intensively studied environmental contaminants. Here we show that an exposure of mice to PAHs induces the expression of Bax in oocytes, followed by apoptosis. Ovarian damage caused by PAHs is prevented by Ahr or Bax inactivation. Oocytes microinjected with a Bax promoter-reporter construct show Ahr-dependent transcriptional activation after PAH, but not dioxin, treatment, consistent with findings that dioxin is not cytotoxic to oocytes. This difference in the action of PAHs versus dioxin is conveyed by a single base pair flanking each Ahr response element in the Bax promoter. Oocytes in human ovarian biopsies grafted into immunodeficient mice also accumulate Bax and undergo apoptosis after PAH exposure in vivo. Thus, Ahr-driven Bax transcription is a novel and evolutionarily conserved cell-death signaling pathway responsible for environmental toxicant-induced ovarian failure.

Inhibition by dietary dibenzoylmethane of mammary gland proliferation, formation of DMBA-DNA adducts in mammary glands, and mammary tumorigenesis in Sencar mice

Dibenzoylmethane (DBM) is a minor constituent of licorice and a beta-diketone analogue of curcumin. Feeding 1% DBM in the diet to Sencar mice during both the initiation and the post-initiation periods strongly inhibited 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumor multiplicity and mammary tumor incidence by 97%. In further in vivo studies to elucidate the possible mechanisms of the inhibitory action of DBM, feeding the 1% DBM in the AIN-76A diet to immature Sencar mice for 4-5 weeks decreased the uterine wet weight by 43%, inhibited the proliferation rate of mammary gland epithelial cells by 53%, uterine epithelium by 23%, and uterine stroma by 77%, when mice were killed during the first estrus phase of estrous cycle. In addition, feeding 1% DBM in the diet to Sencar mice at 2 weeks before, during and 1 week after DMBA treatment (intubation of 1 mg DMBA per mouse once a week for 5 weeks) inhibited formation of total DMBA-DNA adducts in mammary glands by 72% using a post-32P-labeling assay. Thus, feeding 1% DBM diet to Sencar mice inhibited formation of DMBA-DNA adducts in mammary glands and lowered the proliferation rate of the mammary gland in vivo. These results may explain the strong inhibitory actions of dietary DBM on mammary carcinogenesis in mice.

Both (+/-)syn- and (+/-)anti-7,12-dimethylbenz[a]anthracene-3,4-diol-1,2-epoxides initiate tumors in mouse skin that possess -CAA- to -CTA- mutations at Codon 61 of c-H-ras

We have determined the tumor-initiating activity of (+/-)syn- and (+/-)anti-7,12-dimethylbenz[a]anthracene-3,4-diol-1,2-epoxide (syn- and anti-DMBADE), the two metabolically formed bay-region diol epoxides of DMBA, and we have also analyzed mutations in the H-ras gene from tumors induced by these compounds. Using a two-stage, initiation-promotion protocol for tumorigenesis in mouse skin, we have found that both syn- and anti-DMBADE are active tumor initiators, and that the occurrence of papillomas is carcinogen dose dependent. All of the papillomas induced by syn-DMBADE (a total of 40 mice), 96% of those induced by anti-DMBADE (a total of 25 mice), and 94% of those induced by DMBA (a total of 16 mice) possessed a -CAA- to -CTA- mutation at codon 61 of H-ras. No mutations in codons 12 or 13 were detected in any tumor. Topical application of syn- and anti-DMBADE produced stable adducts in mouse epidermal DNA, most of which comigrated with stable DNA adducts formed after topical application of DMBA. Further analysis of the data showed that levels of the major syn- and anti-DMBADE-deoxyadenosine adducts formed after topical application of DMBA are sufficient to account for the tumor-initiating activity of this carcinogen on mouse skin. Previously, we showed that both the syn- and anti-DMBADE bind to the adenine (A182) at codon 61 of H-ras. Collectively, these results indicate that the adenine adducts induced by both bay-region diol epoxides of DMBA lead to the mutation at codon 61 of H-ras and, consequently, initiate tumorigenesis in mouse skin.

The role of polycyclic aromatic hydrocarbon metabolism in dimethylbenz[a]anthracene-induced pre-B lymphocyte apoptosis

Previous studies indicated that two prototypic PAH, benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA), suppress the developing immune system by inducing apoptosis in bone marrow pre-B lymphocytes. In bone marrow cultures consisting of pre-B cells growing on bone marrow stromal cell monolayers, pre-B cell apoptosis was shown to be dependent on the aryl hydrocarbon receptor/transcription factor (AhR) expressed in stromal cells. However, it was not determined if AhR activation alone is sufficient or if DMBA metabolism is required for induction of a stromal cell-derived apoptosis signal. To address these issues we assessed: 1) the ability of poorly metabolized AhR ligands to induce pre-B cell apoptosis and 2) the capacity for and the mechanism through which an early DMBA metabolite induces pre-B cell apoptosis. Three poorly metabolized AhR ligands, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3,3',4,4',5-pentachlorobiphenyl, and 3,3',4,4'-tetrachlorobiphenyl failed to induce pre-B cell apoptosis in bone marrow cultures, indicating that AhR activation alone is not sufficient to induce apoptosis and suggesting a role for PAH metabolism in induction of an apoptosis signal. Consistent with this hypothesis, DMBA-3, 4-dihydrodiol, an early DMBA metabolite, induced significant pre-B cell apoptosis. The ability of DMBA-3,4-dihydrodiol to activate the AhR, inhibition of DMBA-3,4-dihydrodiol-induced apoptosis by alpha-naphthoflavone, and the significantly lower levels of DMBA-3, 4-dihydrodiol-induced apoposis in pre-B cell populations maintained on AhR(-) stromal cells strongly support a role for the AhR in DMBA-3,4-dihydrodiol-induced apoptosis. Of two DMBA-metabolizing enzymes evaluated, CYP1A1 and CYP1B1, the latter appeared to be the more likely to play a role in DMBA-induced apoptosis. These data confirm a role for the AhR in PAH-induced pre-B cell apoptosis, indicate a role for DMBA metabolism, and suggest a feedback loop in which at least one product of DMBA metabolism augments AhR signaling, leading to induction of an apoptosis stimulus.

The flavonoid galangin is an inhibitor of CYP1A1 activity and an agonist/antagonist of the aryl hydrocarbon receptor

The effect of the dietary flavonoid galangin on the metabolism of 7,12-dimethylbenz[a]anthracene (DMBA), the activity of cytochrome P450 1A1 (CYP1A1), and the expression of CYP1A1 in MCF-7 human breast carcinoma cells was investigated. Galangin inhibited the catabolic breakdown of DMBA, as measured by thin-layer chromatography, in a dose-dependent manner. Galangin also inhibited the formation of DMBA-DNA adducts, and prevented DMBA-induced inhibition of cell growth. Galangin caused a potent, dose-dependent inhibition of CYP1A1 activity, as measured by ethoxyresorufin-O-deethylase activity, in intact cells and in microsomes isolated from DMBA-treated cells. Analysis of the inhibition kinetics by double-reciprocal plot demonstrated that galangin inhibited CYP1A1 activity in a noncompetitive manner. Galangin caused an increase in the level of CYP1A1 mRNA, indicating that it may be an agonist of the aryl hydrocarbon receptor, but it inhibited the induction of CYP1A1 mRNA by DMBA or by 2,3,5,7-tetrachlorodibenzo-p-dioxin (TCDD). Galangin also inhibited the DMBA- or TCDD-induced transcription of a reporter vector containing the CYP1A1 promoter. Thus, galangin is a potent inhibitor of DMBA metabolism and an agonist/antagonist of the AhR, and may prove to be an effective chemopreventive agent.

Effect of dietary soy protein isolate, genistein, and 1,4-phenylenebis(methylene)selenocyanate on DNA binding of 7,12-dimethylbenz[a]anthracene in mammary glands of CD rats

We examined whether a soy protein isolate or one of its major components (genistein) influences the initiation stage of carcinogenesis via DNA binding studies of 7,12-dimethylbenz[a]anthracene (DMBA) in liver and mammary tissue of female CD rats. A semipurified high-fat diet (23.5% corn oil) containing the soy protein isolate (10%), genistein (111 ppm), or 1,4-phenylenebis(methylene)selenocyanate (p-XSC) (5 ppm as selenium) as a positive control was fed to 6-week-old virgin female CD rats for 1 week before carcinogen treatment. Neither soy nor genistein affected the extent of DMBA-DNA binding in liver. In mammary tissue, 111 ppm genistein in the diet was more effective than the soy protein isolate, although the latter contains the same amount of genistein, mainly present as a glucoside conjugate. As shown before, p-XSC inhibited DMBA-DNA binding in mammary tissue. Total binding was inhibited because of reduced formation of three major adducts: anti-diol epoxide deoxyguanosine, syn-diol epoxide deoxyadenosine, and anti-diolepoxide deoxyadenosine. Thus, an additional experiment with 111 and 222 ppm of genistein was performed; 222 ppm genistein had a weaker effect than that observed for 111 ppm. Nevertheless, 111 ppm of genistein in the diet appears to inhibit the initiation phase of DMBA-induced rat mammary tumors and may partially account for the reported inhibitory effect of soy against DMBA-induced rat mammary tumors.

Isoflavone genistein inhibits the initiation and promotion of two-stage skin carcinogenesis in mice

Isoflavone genistein is a specific inhibitor of protein tyrosine kinase (PTK) and has been shown to have a variety of anticancer activities in cultured cells and animal models. We report here that genistein significantly inhibits 7,12-dimethylbenz[a]anthracene (DMBA)-initiated and 12-O-tetradecanoyl phorbol-13-acetate (TPA)-promoted skin tumorigenesis in a two-stage carcinogenesis model. In an initiation study, 10 micromol genistein was applied daily to female SENCAR mouse skin for 1 week, followed by initiation with 10 nmol DMBA. Mice were then treated with twice weekly 4 microg TPA. Genistein was shown to reduce tumor incidence and multiplicity in DMBA-initiated skin tumors by approximately 20 (P < 0.05) and 50% (P < 0.01), respectively. Two promotion studies were conducted using CD-1 and SENCAR mice. In experiment 1, CD-1 mice were initiated with 100 nmol DMBA and followed by a twice weekly regimen of 1 and 5 micromol genistein/4 microg TPA. In experiment 2, SENCAR mice were initiated with 10 nmol DMBA and followed by a regimen of 5, 10 and 20 micromol genistein/2 microg TPA. Both studies consistently showed that genistein substantially inhibited TPA-promoted skin tumorigenesis by reducing the tumor multiplicity by approximately 60 and 75%, respectively (P < 0.01). However, the tumor incidence appeared to be less affected. Mechanistic studies showed that genistein inhibited DMBA-induced bulky DNA adduct formation and substantially suppressed TPA-stimulated H2O2 and inflammatory responses in mouse skin by >60% (P < 0.01). In contrast, genistein only exhibited a moderate inhibition of TPA-induced ornithine decarboxylase activity (P > 0.05). Our results suggest that genistein exerts its anti-initiational and anti-promotional effects on skin carcinogenesis probably through blockage of DNA adduct formation and inhibition of oxidative and inflammatory events in vivo.

Diosmin and diosmetin are agonists of the aryl hydrocarbon receptor that differentially affect cytochrome P450 1A1 activity

We investigated the effect of the chemopreventive compound diosmin and its aglycone form, diosmetin, on the carcinogen activation pathway mediated by the aryl hydrocarbon receptor (AhR) in MCF-7 human breast epithelial cancer cells. Treatment of the cells with diosmin caused a dose-dependent increase in the metabolism of the mammary carcinogen 7,12-dimethylbenz(a)anthracene (DMBA), as assessed by increased formation of DMBA-DNA adducts and by DMBA-induced cytotoxicity. In contrast, treatment of the cells with diosmetin decreased both parameters. Diosmetin, but not diosmin, directly inhibited cytochrome P450 1A1 (CYP1A1) activity in a noncompetitive manner in microsomes isolated from DMBA-treated cells, as assayed by ethyoxyresorufin-O-deethylase activity. Treatment of the cells with diosmin or diosmetin, on the other hand, caused a dose- and time-dependent increase in CYP1A1 activity in intact cells that was comparable to that induced by DMBA or by the aryl hydrocarbon benzo(a)pyrene. Both diosmin and diosmetin caused an increase in the transcription of the CYP1A1 gene, as measured by increased levels of CYP1A1 mRNA. Both compounds caused the activation of the DNA-binding capacity of the AhR for the xenobiotic-responsive element of CYP1A1. These results indicate that diosmin and diosmetin are natural dietary agonists of the AhR, causing a potent increase in CYP1A1 transcription and CYP1A1 activity; however, only diosmetin is capable of inhibiting CYP1A1 enzyme activity, thus inhibiting carcinogen activation.

Inhibition by an extract of Ocimum sanctum of DNA-binding activity of 7,12-dimethylbenz[a]anthracene in rat hepatocytes in vitro

Ocimum sanctum is a traditional medicinal plant. Previous studies have shown that extracts of O. sanctum inhibit the induction of skin papillomas in mice by 7,12-dimethylbenz[a]anthracene (DMBA). In the present study, primary cultures of rat hepatocytes were treated with 0-500 microg of O. sanctum extract for 24 h and then with DMBA (10 or 50 microg) for 18 h. Cells were then harvested and their DNA was isolated and analyzed by 32P-postlabelling. A significant reduction in the levels of DMBA-DNA adducts was observed in all cultures pretreated with O. sanctum extract. This effect was more pronounced at the lower dose of DMBA (10 microg). Hepatocytes which were treated with the highest dose of extract (500 microg) showed a maximum reduction of 93% in the mean values of DMBA-DNA adducts. The viability of the cells was not adversely affected by pretreatment with extract. Our findings suggest that O. sanctum leaf extract blocks or suppresses the events associated with chemical carcinogenesis by inhibiting metabolic activation of the carcinogen.

Metabolic activation of methyl-hydroxylated derivatives of 7,12-dimethylbenz[a]anthracene by human liver dehydroepiandrosterone-steroid sulfotransferase

Methyl-hydroxylated metabolites of the potent carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA), namely, 7-hydroxymethyl-12-methylbenz[a]anthracene (7-OH-DMBA), 7-methyl-12-hydroxymethylbenz[a]anthracene (12-OH-DMBA) and 7,12-dihydroxymethylbenz[a]anthracene (7,12-diOH-DMBA), were examined as substrates for sulfotransferase bioactivation in different human tissue cytosols. Hepatic cytosols, which were able to catalyze the 3'-phosphoadenosine 5'-phosphosulfate (PAPS)-dependent DNA binding of 7-OH-DMBA, 12-OH-DMBA and 7,12-diOH-DMBA, were highly sensitive to inhibition by dehydroepiandrosterone (DHEA), a specific substrate for human DHEA-steroid sulfotransferase (IC50 = 5 microM). By comparison, 2,6-dichloro-4-nitrophenol, a potent inhibitor of the thermostable (TS)-phenol and estrogen sulfotransferases, did not have an appreciable inhibitory effect. Neither p-nitrophenol, a high affinity substrate for human TS-phenol and estrogen sulfotransferases, nor dopamine, a specific substrate for the thermolabile (TL)-phenol sulfotransferase, significantly inhibited the DNA binding of 12-OH-DMBA catalyzed by hepatic cytosols. Inter-subject variation (n = 12) of the PAPS-dependent DNA binding of 12-OH- and 7,12-diOH-DMBAs also correlated well with DHEA-sulfotransferase activity (r = 0.90; P < 0.00001 and r = 0.92; P < 0.00001, respectively). This sulfation-dependent metabolic activation was not detected in cytosols from human colon, pancreas, larynx or mammary gland. Both TS- and TL-phenol sulfotransferases were active in human liver and colon but only liver contained DHEA-sulfotransferase activity. These results indicate that the sulfotransferase-mediated activation of the methyl-hydroxylated DMBAs is predominantly catalyzed by DHEA-steroid sulfotransferase in human liver and that TS- and TL-phenol sulfotransferases and estrogen sulfotransferase are not involved in the catalysis.

Comparison of immunoaffinity chromatography enrichment and nuclease P1 procedures for 32P-postlabelling analysis of PAH-DNA adducts

32P-postlabelling analysis for detecting DNA adducts formed by polycyclic aromatic compounds is one of the most widely used techniques for assessing genotoxicity associated with these compounds. In cases where the formation of adducts is extremely low, a crucial step in the analysis is an enrichment procedure for adducts prior to the radiolabelling step. The nuclease P1 enhancement procedure is the most established and frequently used of these methods. An immunoaffinity procedure developed for class specific recognition for polycyclic aromatic hydrocarbon (PAH)-DNA adducts has therefore been compared with the nuclease P1 method for a range of DNA adducts formed by PAHs. The evaluation was carried out with skin DNA from mice treated topically with benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, 5-methylchrysene or chrysene. The immobilised antibody had the highest affinity for adducts structurally similar to the BPDE-I-deoxyguanosine adduct ([+/-]-N2-(7r,8t,9r-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene-1 0t-yl)-2'-deoxyguanosine) against which the antibody had been raised. Of the PAH-modified DNAs evaluated, the maximum adduct recovery was obtained for DNA containing the BPDE I-deoxyguanosine adduct. With DMBA-modified DNA, the profiles of adducts recovered from the column were similar when the column material was treated either with a digest of DMBA-modified DNA or with 32P-labelled DMBA adducts. I-compounds (endogenous adducts in tissue DNA of unexposed animals), which had similar chromatographic properties to PAH-DNA adducts, were not enriched by the immunoaffinity procedure. Compared to the simple nuclease P1 enhancement procedure, the immunoaffinity methods were lengthier and more labour intensive. Advantages of the immunoaffinity procedure include: specificity, allowing the selective detection of a certain class of adducts: efficient adduct enrichment, providing a viable alternative to other enrichment procedures; adequate sensitivity for model studies and the potential to purify adducts for further characterisation. However, as a general screen for detecting the formation of DNA adducts, the nuclease P1 procedure was viewed as the initial method of choice since it was capable of detecting a wider range of PAH-DNA adducts.

Genistein alters the ontogeny of mammary gland development and protects against chemically-induced mammary cancer in rats

Breast cancer is the most common cancer in US females and is the second leading cause of cancer death among women. By contrast, Asian women consuming a traditional diet high in soy products have a relatively low incidence of breast cancer. Asians who emigrate to the United States and adopt a Western diet lose this protection. Soy-based diets are high in phytoestrogens, and one of these components is genistein. Using the dimethylbenz(a)anthracene (DMBA) mammary cancer rodent model, we have investigated the breast cancer protective potential of genistein. Our results demonstrate that neonatal and prepubertal genistein treatments altered the ontogeny of the mammary gland and rendered the adult animals less susceptible to chemically-induced mammary cancer. Neonatal genistein treatment did not significantly alter the rate of formation and persistence of DMBA-DNA adducts in the mammary gland. While high concentrations of genistein during the neonatal period caused adverse effects on ovarian follicular development, prepubertal genistein treatment did not appear to be toxic in either the female reproductive tract or the endocrine system.

Tissue distribution of DNA adducts in rats treated by intramammillary injection with dibenzo[a,l]pyrene, 7,12-dimethylbenz[a]anthracene and benzo[a]pyrene

Dibenzo[a,l]pyrene (DBP) has recently emerged as a potent environmental carcinogen having greater carcinogenicity in the rat mammary epithelial glands than 7,12-dimethylbenz[a]anthracene (DMBA), previously considered to be the most potent mammary carcinogen and benzo[a]pyrene (BP), a ubiquitous environmental carcinogen. Previous studies on the tumor-initiating potential of DBP, DMBA, and BP demonstrated that DBP was 2.5 times more potent in inducing the tumors in mouse skin and rat mammary glands than DMBA; BP was a weak mammary carcinogen in these animals. The present study was designed to investigate if the significantly increased mammary carcinogenicity of DBP over DMBA and BP was related to increased DNA adduction at the target site. Female Sprague-Dawley rats were treated by intramammillary injection with an equimolar dose of 0.25 micromol/gland of DBP, DMBA, and BP at the 3rd, 4th and 5th mammary glands on both sides. 32P-Postlabeling analysis of mammary epithelial DNA of rats treated with DBP produced two major (nos. 3 and 6) and at least 5 minor adducts. DMBA treatment resulted in one major and 4 minor DNA adducts while BP produced one major and two minor adducts. Quantitation of the adduct radioactivity revealed that DNA adduction was 6- and 9-fold greater in DBP-treated animals than in BP- and DMBA-treated animals, respectively. The adduct levels per 10(9) nucleotides in mammary epithelial cells for DBP, BP and DMBA were in the following descending order: 1828 +/- 378, 300 +/- 45 and 207 +/- 72, respectively. Tissue distribution of DNA adducts in non-target organs following DBP treatment showed similar adduct pattern as found in the mammary epithelial cells except the liver, which resulted in 4 additional adduct spots; vehicle-treated tissue DNA processed in parallel did not show any detectable adducts. DMBA- and BP-DNA adduct patterns in various tissues were similar to that found in mammary epithelial cells, however, significant quantitative differences were found; BP-DNA adducts were undetectable in the pancreas and bladder. Quantitation of adduct radioactivity showed a 15- to 60-fold lower DBP-DNA adduction in these tissues than the levels found in the mammary tissue; similarly 5-20 and 30-100 times lower DNA adduction was found following treatment with DMBA and BP, respectively. The significantly increased binding of DBP to the mammary epithelial DNA over BP and DMBA is in concordance with its known higher mutagenicity and tumorigenicity.

32P-Postlabeling high-performance liquid chromatographic improvements to characterize DNA adduct stereoisomers from benzo[a]pyrene and benzo[c]phenanthrene, and to separate DNA adducts from 7,12-dimethylbenz[a]anthracene

Single compounds can generate complex DNA adduct patterns by reactions through different pathways, with different target nucleotides and through different configurations of the products. DNA adduct analysis by 32P-HPLC was improved by adding an isocratic plateau in an otherwise linear gradient, thereby enhancing resolution of predictable retention time intervals. This enhanced 32P-HPLC technique was used to analyze and at least partly resolve 14 out of 16 available benzo[c]phenanthrene deoxyadenosine and deoxyguanosine adduct standards, 8 out of 8 available benzo[a]pyrene deoxyadenosine and deoxyguanosine adduct standards, and 51 peaks from 7,12-dimethylbenz[a]anthracene-calf thymus DNA reaction products. The same type of gradient modifications could be used to enhance resolution in analyses of other complex DNA adduct mixtures, e.g., in vivo in humans.

Nitric oxide synthase and skin tumor promotion

Up-regulation of inducible form of nitric oxide (NO)-synthase and increased production of NO has been shown to occur in many pathological conditions associated with inflammatory responses. In this study we show that topical application of skin tumor promoters, which are known to produce inflammatory response, down-regulate the constitutive form of NO-synthase in SENCAR mouse epidermis. The phorbol type of tumor promoters viz 12-0-tetradecanoylphorbol-13-acetate (TPA) and mezerein produced greater inhibitory effects than the non phorbol tumor promoters viz anthralin, n-dodecane, benzoyl peroxide and okadaic acid. Pretreatment of the skin with a polyphenolic fraction isolated from green tea, which possesses strong antioxidant activity, almost completely restored the inhibitory response. We suggest that the constitutive NO-synthase activity may play an important role in tumor promoter-caused oxidative burst during the promotion stage of multistage skin carcinogenesis, and that antioxidants may also target their anti-tumor promoting effect via attenuating the NO-synthase pathway.

7-Sulfooxymethyl-12-methylbenz[a]anthracene is an exceptionally reactive electrophilic mutagen and ultimate carcinogen

The hypothesis was tested that an ultimate carcinogen of 7-hydroxymethyl-12-methylbenz[a]anthracene (HMBA), a major metabolite of 7,12-dimethylbenz[a]anthracene (DMBA), is a benzylic carbonium ion generated from an exceptionally reactive aralkylating metabolite, such as an electrophilic sulfate ester. In conformity with this hypothesis, sarcomas were rapidly induced in rats following repeated subcutaneous injection of HMBA (67%) or its electrophilic sulfate ester, sodium 7-sulfooxymethyl-12-methylbenz[a]anthracene (SMBA) (100%). It would appear from the results summarized here that the search for a carcinogenic metabolite of DMBA has been successful. In addition, an aralkylating electrophilic mutagen and carcinogen has been prepared from HMBA, which is itself either an ultimate carcinogen or a direct precursor of an ultimate carcinogen, i.e., a benzylic carbonium ion.

Inhibitory effects of naturally occurring coumarins on the metabolic activation of benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene in cultured mouse keratinocytes

Several naturally occurring coumarins to which humans are routinely exposed have been previously found to be potent inhibitors and inactivators of cytochrome P450 (P450) 1A1-mediated monooxygenase in both murine hepatic microsomes and in a reconstituted system using purified human P450 1A1 [Cai et al. (1993) Chem. Res. Toxicol., 6, 872-879 and Cai et al. (1996) Chem. Res. Toxicol., 9, 729-736]. In the present study, several of these coumarins were investigated for their inhibitory effects on the metabolism and metabolic activation of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA) in cultured mouse keratinocytes. Initial analysis of B[a]P metabolism in cultured keratinocytes showed that imperatorin, isoimperatorin, coriandrin, and bergamottin, at concentrations of 2 nM equal with B[a]P, reduced the formation of water-soluble metabolites of B[a]P by 33% to 57%. Bergamottin and coriandrin were the most potent inhibitors of the compounds examined. HPLC analysis of organic solvent-soluble metabolites of B[a]P indicated that all the coumarins tested significantly reduced the formation of individual B[a]P metabolites (including phenols, diols and tetraols). However, the greatest effect was on the formation of B[a]P tetraols. Additional experiments determined the ability of selected coumarins to block covalent binding of B[a]P and DMBA to DNA in keratinocytes. Bergamottin preferentially inhibited the binding of B[a]P to DNA by 56%, while coriandrin preferentially inhibited the binding of DMBA to DNA by 48%. Notably, analysis of individual DNA adducts formed from B[a]P and DMBA indicated that both bergamottin and coriandrin specifically inhibited the formation of anti diol-epoxide DNA adducts derived from both hydrocarbons. The preferential inhibitory effect of bergamottin and coriandrin on the formation of anti diol-epoxide adducts derived from DMBA was further confirmed by separation of anti- and syn-diol-epoxide-DNA adducts using immobilized boronate chromatography. The current study demonstrates that certain naturally occurring coumarins inhibited metabolic activation of B[a]P and DMBA in cultured mouse keratinocytes and specifically inhibited the formation of DNA adducts derived from the anti diol-epoxide diastereomers from either hydrocarbon. The current data also suggest that certain naturally occurring coumarins may possess anticarcinogenic activity toward polycyclic aromatic hydrocarbons.

Synthesis and structure determination of the adducts formed by electrochemical oxidation of 1,2,3,4-Tetrahydro-7,12-dimethylbenz[a]anthracene in the presence of deoxyribonucleosides or adenine

Study of DNA adducts formed with aromatic hydrocarbons is part of the strategy to elucidate the mechanisms of tumor initiation by these compounds. 1,2,3,4-Tetrahydro-7,12-dimethylbenz[a]anthracene (THDMBA) is of special interest because it allows discrimination between the pathways of bioactivation by one-electron oxidation and monooxygenation. To study and identify adducts formed biologically, synthetic adducts are needed as reference standards. THDMBA was electrochemically oxidized in the presence of deoxyadenosine (dA), adenine (Ade), deoxyguanosine (dG), or deoxycytidine (dC). In the presence of dA, four adducts were isolated: 7-methyl-1,2,3,4-tetrahydrobenz[a]anthracene-12-CH2-N7Ade (7-MTHBA-12-CH2-N7Ade, 3.6%), 12-MTHBA-7-CH2-N7Ade (4.2%), 7-MTHBA-12-CH2-N6dA (5.8%), and 12-exo-methylene-7-MTHBA-7-N6dA (22.8%); a dehydrogenated product, 7,12-di-exo-methylene-THBA (44.2%), was also obtained. In the presence of Ade, nine adducts were synthesized: 7-MTHBA-12-CH2-N7Ade (1.1%), 12-MTHBA-7-CH2-N7Ade (2.4%), 7-MTHBA-12-CH2-N1Ade (10.2%), 12-MTHBA-7-CH2-N1Ade (13.2%), 7-MTHBA-12CH2-N3Ade (1.7%), 12-MTHBA-7-CH2-N3Ade (1.7%), 7-exo-methylene-12-MTHBA-12-N3Ade (11.2%), 12-exo-methylene-7-MTHBA-7-N3Ade (27.9%), and 12-exo-methylene-7-MTHBA-7-N6Ade (12.1%), as well as the dehydrogenated product 7,12-di-exo-methylene-THBA (16.7%). In the presence of dG, three adducts were produced: 7-MTHBA-12-CH2-N7Gua (24.2%), 12-MTHBA-7-CH2-N7Gua (12.2%), and 7-MTHBA-12-CH2-N2dG (3.7%), as well as the dehydrogenated product 7,12-di-exo-methylene-THBA (38.9%). Anodic oxidation in the presence of dC yielded a large amount of 7,12-di-exo-methylene-THBA (80.4%), but no adducts. The structure of the adducts was elucidated by using UV, NMR, and MS. The N-7 positions in dG, dA, and Ade, the 2-NH2 in dG, and the N-1 position in Ade form exclusively methyl-linked adducts. In contrast, the 6-NH2 group of dA and Ade and the N-3 of Ade prefer to attack the meso-anthracenic positions rather than the methyl groups. The order of reactivity of dG and dA in the formation of methyl-linked THDMBA adducts agrees well with that previously found for 7,12-dimethylbenz[a]anthracene [RamaKrishna et al. (1992) J. Am. Chem. Soc. 114, 1863-1874.

Sequence preference of 7,12-dimethylbenz[a]anthracene-syn-diol epoxide-DNA binding in the mouse H-ras gene detected by UvrABC nucleases

We have found that 7,12-dimethylbenz[a]anthracene-syn-diol epoxide (syn-DMBADE)-modified DNA fragments are sensitive to UvrABC incision. The incisions occur mainly seven bases 5' and four bases 3' of a syn-DMBADE-modified adenine or guanine residue. The kinetics of UvrABC incision at different sequences in a DNA fragment are the same, and the extent of UvrABC incision is proportional to the syn-DMBADE concentration. On the basis of these results, we have concluded that UvrABC incision on syn-DMBADE-DNA adducts is independent of DNA sequence and is quantitative. Using the UvrABC incision method, we have analyzed the syn-DMBADE-DNA binding spectrum in several defined DNA fragments, including the first two exons of the mouse H-ras gene. We have found that both guanine and adenine residues in codons 12, 13, and 61 of the H-ras gene are strong syn-DMBADE binding sites. These results suggest that the initial binding of DMBADE may greatly contribute to the frequency of H-ras mutations. Results from dinucleotide binding analysis indicate that the 5'-nearest neighbor displays a greater effect on syn-DMBADE-DNA binding than the 3'-nearest neighbor.

Inhibition by rosemary and carnosol of 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumorigenesis and in vivo DMBA-DNA adduct formation

Extracts of the spice Rosemary officinalis L. have been reported to inhibit experimental carcinogenesis. Two rosemary components, carnosol and ursolic acid, appear to be partly responsible for the antitumorigenic activity of rosemary. The present studies were conducted in order to evaluate the activity of rosemary extract, carnosol and ursolic acid in inhibiting the in vivo formation of mammary 7,12-dimethylbenz[a]anthracene (DMBA)-DNA adducts and the initiation of DMBA-induced mammary tumorigenesis in female rats. Supplementation of diets for 2 weeks with rosemary extract (0.5% by wt) but not carnosol (1.0%) or ursolic acid (0.5%) resulted in a significant decrease in the in vivo formation of rat mammary DMBA-DNA adducts, compared to controls. When injected intraperitoneally (i.p.) for 5 days at 200 mg/kg body wt, rosemary and carnosol, but not ursolic acid, significantly inhibited mammary adduct formation by 44% and 40%, respectively, compared to controls. Injection of this dose of rosemary and carnosol was associated with a significant 74% and 65% decrease, respectively, in the number of DMBA-induced mammary adenocarcinomas per rat, compared to controls. Ursolic acid injection had no effect on mammary tumorigenesis. Therefore, carnosol is one rosemary constituent that can prevent DMBA-induced DNA damage and tumor formation in the rat mammary gland, and, thus, has potential for use as a breast cancer chemopreventative agent.

Dose-time response in mouse skin tumor induction by 7, 12-dimethylbenz[a]anthracene and 12-O-tetradecanoyl-phorbol-13-acetate

The question was addressed whether the dose-response relationship derived from a carcinogenicity study can be used for mechanistic interpretation and to what extent the shape of the curve is dependent on the duration of the bioassay and the time of analysis. The mouse skin tumor model was used. It allows recording of the time of tumor appearance without interim sacrifice. Groups of 16 female NMRI mice were treated twice weekly by dermal administration with combinations of (a) 2.5 nmol 12-O-tetradecanoyl-phorbol-13-acetate (TPA) plus 0, 0.3, 1, 3, or 10 nmol 7,12-dimethylbenz[a]anthracene (DMBA) or with (b) 2.5 nmol DMBA plus 0, 0.1, 0.3, 1, 3, or 10 nmol TPA. The appearance of the first papilloma was recorded for each animal and the cumulative incidence data were analyzed in two ways: (i) With the usual dose-prevalence representation at a fixed time point, the dose response for TPA was sigmoidal, while it was linear-superlinear for DMBA. This was observed at all time points, indicating that the dose-response information may be used for a distinction between DNA-reactive and tumor-promoting mechanisms of action. (ii) When time-to-tumor and loss of tumor-free lifetime was analyzed as a function of dose, there was again a marked difference between DMBA and TPA. The tumor-free lifetime increased with each step of dose reduction but the slope was about four times larger for TPA compared with that for DMBA. Further reduction of the TPA dose could result in a situation in which the natural life span sets a limit to an observable effect. Under the conditions of this bioassay for mouse skin papilloma induction by a combination treatment with DMBA plus TPA, the findings support the idea of a no-effect low-dose threshold for the tumor-promoting agent.

Evanescent fluorobiosensor for the detection of polyaromatic hydrocarbon based on DNA intercalation

A flow-injection analysis (FIA) system coupled with an evanescent wave (EW) biosensor employing total internal reflection of fluorescence radiation (TIRF) for the detection of polyaromatic hydrocarbon that intercalates into DNA is reported. A highly fluorescent intercalator, "ethidium bromide," has been used as the reference compound for detection. The EW biosensor was developed according to the procedure described earlier (1,2). Data on the analysis of Naphthalene, 3-methylcholanthrene, 7,12-dimethylbenz(a)anthracene, 1,2-benzanthracene, and some standard reference materials supplied by the National Institute of Standards and Technology are reported. The relative ability of the polyaromatic hydrocarbon to displace ethidium bromide, based on the relative binding ratio, is found to be on the order of 7,12-dimethylbenz[a]anthracene > 3-methylcholanthrene > 1,2-benzanthracene > napthalene.

Efficacy of cancer prevention by high-selenium garlic is primarily dependent on the action of selenium

We reported previously that garlic cultivated with selenite fertilization showed powerful chemopreventive activity in the rat dimethylbenz[a]anthracene (DMBA)-induced mammary tumor model (Carcinogenesis 15, 573-576, 1994). In order to ascertain that the efficacy of the high-selenium garlic in cancer protection is primarily dependent on the action of selenium we compared the effects of two batches of garlic powder with marked differences in their level of selenium enrichment, 112 or 1355 p.p.m. Se dry weight. Both products were added to the diet to achieve the same final concentration of 2 p.p.m. Se. The supplementation protocol was designed to evaluate the efficacy during either the initiation phase or post-initiation phase of DMBA mammary carcinogenesis. Significant tumor reduction was observed with either treatment protocol. Furthermore, the magnitude tumor suppression, as well as the extent of DMBA-DNA adduct inhibition, were very similar with the two batches of garlic, even though the amounts of garlic in the diet varied considerably between them (1.8% for the 112 p.p.m. Se garlic versus 0.15% for the 1355 p.p.m. Se garlic). This suggests that the anti-cancer activity of the high-selenium garlic was likely to be accounted for by the effect of selenium, rather than the effect of garlic per se. A continuous feeding of the high-selenium garlic produced a modest increase in total selenium in various tissues. In general the profile of selenium accumulation was comparable in rats ingesting either the 112 or the 1355 p.p.m. Se garlic. Thus, based on the results of several biological responses, it appears that the ability of the high-selenium garlic to protect against tumorigenesis is primarily dependent on increased intake of selenium provided by the vegetable. Future research will be focused on the chemical form of selenium in the garlic.

Stereoselectivity of activation of 7,12-dimethylbenz[a]anthracene- 3,4-dihydrodiol to the anti-diol epoxide metabolite in a human mammary carcinoma MCF-7 cell-mediated V79 cell mutation assay

7,12-Dimethylbenz[a]anthracene (DMBA), one of the most carcinogenic polycyclic aromatic hydrocarbons in rodent bioassays, is metabolically activated in many tissues to "bay-region" DMBA-3,4-diol-1,2-epoxides (DMBADE). Unlike benzo[a]pyrene, for which the high biological activity of the (7R,8S)-diol-(9S,10R)-epoxide has been established, the low chemical stability of anti-DMBADE has made it impossible to evaluate the role of specific stereoisomers in the biological activity of DMBA. In order to characterize the role of formation of DMBADE diastereomers in the induction of mutations, postlabeling assays using [35S]phosphorothioate with adduct separation by HPLC and immobilized boronate chromatography analyses were developed to allow separation and quantitation of DNA adducts formed from each stereoisomer of DMBADE. In DMBA-treated hamster embryo cell cultures, large quantities of three major adducts (anti-DMBADE-deoxyguanosine, anti-DMBADE-deoxyadenosine, and syn-DMBADE-deoxyadenosine) along with five minor adducts were completely resolved and quantitated. The DNA isolated from a human mammary carcinoma MCF-7 cell-mediated V79 cell mutation assay treated with increasing doses of racemic DMBA-3,4-dihydrodiol contained large amounts of two anti-DMBADE-DNA adducts. The anti-DMBADE adducts accounted for more than 90% of the total adducts at all doses. The number of 6-thioguanine-resistant mutants was proportional to the amount of anti-DMBADE-DNA adducts.(ABSTRACT TRUNCATED AT 250 WORDS)

Cancer chemoprevention by aliphatic selenocyanates: effect of chain length on inhibition of mammary tumors and DMBA adducts

The objective of this study was to examine the anticarcinogenic activity of a series of aliphatic selenocyanates with increasing length of the carbon side chain, CH3-(CH2)n-SeCN, in which n = 0, 2, 4 or 6. Their ability to prevent mammary cancer was evaluated during the initiation phase using the rat 7,12-dimethyl-benz[a]anthracene (DMBA) model. Each compound was added to the diet at a final concentration of 2 p.p.m. Se and was given from 2 weeks before to 1 week after DMBA administration. Analysis of the tumor data suggested the following order of chemopreventive potency for this series of aliphatic selenocyanates: heptyl approximately pentyl > propyl > methyl. Thus it appears that the length of the carbon side chain is a determinant in modulating the efficacy of these selenium homologs. In vivo results of total DMBA binding and adduct formation in the mammary cells showed a similar trend of progressive reductions following treatment by selenocyanates with increasing length of the alkyl side chain. These studies strongly indicate that aliphatic selenocyanates are effective blocking agents in the DMBA model and are capable of modulating events in the initiation phase of mammary carcinogenesis.

Comparison of 7,12-dimethylbenz(a)anthracene-DNA adduction in the epidermis of two lines of mice selected for resistance (CAR-R) or susceptibility (CAR-S) to skin carcinogenesis

Two lines of mice were produced by bidirectional selective breeding: one resistant (CAR-R) and one susceptible (CAR-S) to two-stage skin carcinogenesis by dimethylbenz(a)anthracene and 12-O-tetradecanoyl-phorbol-13-acetate. The dimethylbenz(a)anthracene-DNA adduct formation was compared in the two lines by a postlabeling procedure so as to determine whether the striking interline difference observed as to tumor incidence could (in part) be due to differences in the formation of DNA-reactive metabolites. Results show that qualitatively, adduct profiles in CAR-R and CAR-S epidermis are similar. Quantitatively, the total binding level is slightly higher in CAR-S versus CAR-R mice during the 30-day follow-up. However, these minor differences do not increase in function of the response to selection observed through three consecutive generations. A 2- or 4-week promotion with 12-O-tetradecanoylphorbol-13-acetate enhances the decrease of adduct level in the two lines. This effect is somewhat more pronounced in CAR-S mice. Results strongly suggest that the expression of the genes responsible for CAR-R/CAR-S phenotypic difference affects mainly the postinitiation stages.

Impact of various sources of garlic and their constituents on 7,12-dimethylbenz[a]anthracene binding to mammary cell DNA

The present studies assessed the impact of various sources of garlic and their constituents (water- and ethanol-extracts and S-allylcysteine) on the in vivo binding of the carcinogen 7,12-dimethylbenz[a]anthracene (DMBA) to rat mammary cell DNA. The provision of dietary raw garlic powder (2%) or its water-extract (1.5%) reduced DMBA-DNA binding by 33 and 46% respectively. Dietary supplementation with a commercially available deodorized garlic powder (powder A) at 2 or 4% depressed the occurrence of adducts by 50 and 78% respectively, while providing a commercially available high sulfur garlic preparation (powder B) at 2% reduced binding by 56%. A pair-feeding study revealed that the depression in carcinogen binding was independent of food intake or weight gain. Although 1% raw garlic powder did not significantly influence the occurrence of DMBA-DNA adducts, an equivalent as the water-extract (0.75%), the ethanol-extract (0.015%) or commercially available powders (A and B) reduced DMBA adducts in mammary tissue by 44, 25, 71 and 65% respectively. Dietary fortification with S-allylcysteine (SAC), a water-soluble constituent of processed garlic, caused a progressive decrease in the binding of DMBA to DNA. Studies with SAC suggest the primary effect of garlic and its constituents is on the bioactivation and binding of the carcinogen rather than DNA repair. These data reveal that several forms of garlic are effective, although variable, in altering carcinogen bioactivation and presumably chemically induced carcinogenesis.

Time-varying conjugation of 7,12-dimethylbenz[a]anthracene metabolites in rainbow trout (Oncorhynchus mykiss)

Sexually immature rainbow trout were dosed via gavage with 7,12-[14C]dimethylbenz[a]anthracene (DMBA) in order to study hepatic metabolite formation over time and at single and multiple doses. beta-Glucuronidase and aryl sulfatase hydrolyses of bile extracts and subsequent high-pressure liquid chromatography analysis demonstrated significant levels of sulfate conjugates formed after 12 hr of exposure to a single dose. By 72 hr, glucuronide conjugates had increased and after three doses of [14C]DMBA, glucuronides exceeded the amount of sulfates by almost an order of magnitude. Of the metabolites that could be tentatively identified, the majority were oxidative derivatives of the aromatic rings 3-OH DMBA and 3,4-trans-OHDMBA, the latter of which is mutagenic and a precursor to the proposed ultimate carcinogen of DMBA.